As an element member to be used in a semiconductor device, photosensitive device for use in electrophotography, image-reading sensor, image-pickup device, photoelectromotive force member, etc., there have been proposed a film composed of an amorphous material containing silicon atoms as the main constituent (hereinafter referred to as "A-Si") and another kind of film which is composed of an amorphous material containing silicon atoms as the main constituent and hydrogen atoms (hereinafter referred to as "A-Si:H").
Along with the proposals of such films, there have also been proposed a number of methods by means of vacuum evaporation, thermal induced chemical vapor deposition, plasma chemical vapor deposition (plasma CVD), reactive sputtering, ion plating and light induced chemical vapor depositions for preparing such films and also, a number of apparatuses for practicing said methods.
Among those methods, the method of plasma CVD has been generally evaluated as being the most preferred and is currently used to prepare said functional deposited films on a commercial basis.
By the way, according to the method of plasma CVD, a functional deposited film is prepared by exciting film forming raw material gases with an excitation energy of direct current or microwave to generate active species capable of contributing to formation of said film in the reaction space near a substrate in a deposition chamber, which concurrently react each other to result in forming said functional deposited film on said substrate.
And there have been proposed a number of apparatuses for practicing the method of plasma CVD.
FIG. 3 is a diagrammatic illustration of a known typical apparatus for forming a deposited film on a cylindrical substrate by means of plasma CVD.
The apparatus illustrated in FIG. 3 includes a cathode 1 which serves as a surrounding wall of deposition chamber having a film forming space, a cylindrical substrate 2' disposed on a substrate holder 2, having an electric heater 8 and being mechanically connected through a rotary shaft 9 to a motor (not shown) to rotate the substrate 2' during film forming operation, which is grounded so as to make the substrate 2' to act as an anode, upper wall 3 and bottom wall 13 respectively of the deposition chamber, both of which are insulated from the cathode 1 by electrical porcelains 4. In FIG. 3, reference numeral 5 stands for a high frequency power source which is electrically connected to a raw material gas supplying pipe 6 extended from gas reservoirs (not shown) which is connected to the surrounding wall (cathode) 1 and open into the space formed by the inner face of the surrounding wall (cathode) 1 and an inside wall 11 having a number of gas liberation holes 10, 10, --to allow raw material gas to be fed dispersedly against the surface of the substrate 2' in the film forming space. The bottom wall 13 is provided with an exhaust pipe 7 which is connected through an exhaust valve (not shown) to a vacuum pump (not shown).
FIG. 4 is a diagrammatic illustration of another known typical apparatus for forming a deposited film on a cylindrical substrate by means of plasma CVD, which is a partial modification of the apparatus illustrated in FIG. 3. In FIG. 4, the same reference numerals as in FIG. 3 respectively mean the same members of the apparatus illustrated in FIG. 3.
In the apparatus illustrated in FIG. 4, raw material gas is fed dispersedly against the surface of the substrate 2' through gas liberation holes 10, 10, --of each of plural gas feed pipes 12 installed vertically and in parallel with the substrate 2' in the film forming space of the deposition chamber.
Operation of forming a deposited film on a substrate in such apparatus as above mentioned by means of plasma CVD is practiced as below described.
That is, gases in the deposition chamber are exhausted and the inner atmosphere of the deposition chamber is vacuumed through the exhaust pipe 7 to a predetermined vacuum. Successively, the cylindrical substrate 2' is heated to and kept at a predetermined temperature by activating the heater 8, and it is rotated by means of the motor. Then, film forming raw material gas, for instance, silane gas in the case of forming a deposited film composed of an amorphous material containing silicon atoms and hydrogen (hereinafter referred to as "A-Si:H"), is fed through the gas liberation holes 10, 10, --either of the inside wall 11 in case of the apparatus illustrated in FIG. 3 or of the gas feed pipes 12 in case of the apparatus illustrated in FIG. 4 into the film forming space of the deposition chamber. At the same time, a high frequency power from the high frequency power source 5 is impressed between the cathode 1 and the substrate 2' (anode) to thereby cause plasma discharging in the film forming space, which excites the raw material gas to generate radical particles such as Si*, SiH* etc. (the symbol * means an excited state), electrons and ion particles thereof which cause formation of a deposited film on the substrate 2'.
In the above mentioned operation of forming a deposited film, the thickness and the quality of a deposited film to be formed is influenced by the pressure and the flow rate of a film forming raw material gas to be introduced into the deposition chamber, the discharging power to form gas plasmas in the film forming space, etc. And in particular, the distribution state of the strength in plasma discharge will be an important factor in order to a desired deposited film of uniform film quality which has an uniform layer thickness. However, in an apparatus of the type of either FIG. 3 or FIG. 4 for forming a deposited film in accordance with the method of plasma CVD, there are still left problems as being unsolved that the distribution state of the strength in plasma discharge often becomes uneven from upper portion through middle portion to lower portion in the film forming space of the deposition chamber to thereby make the resulting deposited film uneven in its quality and layer thickness accordingly. And, the longer the cylindrical substrate becomes, the more said problems becomes significant.
Against this background, there is now an increased demand for making appropriate improvements in such apparatus as above mentioned for forming a deposited film in accordance with the method of plasma CVD so that a desired deposited film of uniform and high film quality which has an uniform layer thickness can be stably and effectively obtained without occurrence of the above mentioned problems in a simple apparatus.